Increasing price of energy sources due to depletion of fossil fuels and an increased interest in environmental pollution have brought about increased demand for environmentally friendly alternative energy sources as an indispensable element for future life. Studies on various power generation technologies such as nuclear, solar, wind, and tidal power generation technologies have continued to be conducted and power storage devices for more efficient use of such generated energy also continue to be of great interest. Secondary batteries have been used as such power storage devices. Among secondary batteries, lithium secondary batteries have begun to be used for mobile devices and, along with increasing demand for reduced weight and high voltage and capacity, recently, use of lithium secondary batteries has been significantly extended to electric vehicles, hybrid electric vehicles, and auxiliary power sources based upon smart grid.
However, numerous challenges, which have yet to be addressed, remain before lithium secondary batteries can be used as large-capacity power sources. One important challenge is to improve energy density and increase safety. Another important challenge is to reduce process time and to achieve uniform wetting for large-area electrodes. Many researchers have conducted intensive studies on materials that can satisfy low cost requirements while increasing energy density and have also put effort into studies on materials for improving safety.
Ni-based or Mn-based materials having higher capacity than LiCoO2, which has been conventionally used, are typical examples of materials being studied for energy density improvement. Materials that are based on Li alloying reactions with Si or Sn rather than based on intercalation reactions are typical examples of materials for anodes being studied as alternatives to conventional graphite-based materials.
A stable olivine-based cathode active material such as LiFePO4, a cathode active material such as Li4Ti5O12, or the like have been studied to improve safety. However, such materials for safety improvement inherently have a low energy density and do not fundamentally solve safety problems associated with the structure of lithium secondary batteries.
Secondary battery safety may be largely divided into internal safety and external safety and may further be divided into electrical safety, impact safety, and thermal safety. Occurrence of safety problems commonly entails temperature increase, which necessarily results in contraction of a stretched separator that is generally used.
Although some batteries use an unstretched solid electrolyte to electrically separate the cathode and the anode from one another, the batteries do not provide desired battery performance, for example, due to limited ionic conductivity of the solid.
Thus, there is a great need to provide a battery structure that prevents short-circuiting due to separator contraction and provides excellent battery performance.